Probiotics are live microorganisms which, when administered in adequate amounts, confer a health benefit on the host. Although Probiotics have a long history of traditional use, an ever increasing range of probiotic organisms are being sold as ingredients in foods or nutritional supplements. Despite their widespread use to maintain health and prevent or treat diseases, little is known about the safety of probiotics and their mechanisms of action, particularly their effects on the human-associated microbial communities and on the host. To start to address this gap in knowledge, we propose to conduct a Phase I, double-blind randomized controlled clinical trial to evaluate the safety, tolerability and effect on the oropharyngeal and gut microbiota of Lactobacillus GG ATCC 53103 (LGG), versus placebo, taken daily for 6 months, in 50 normal healthy adult volunteers. This study will be conducted under an Investigational New Drug (IND) application from the Food and Drug Administration. Our specific aims are to assess (1) the safety and tolerability of twice daily dosing of LGG and (2) whether LGG colonizes the oropharynx and/or GI tract of healthy adult volunteers and to assess the effect of LGG on the microbiota of the oropharynx and gut during the study period. Safety assessment and metagenomic evaluation, including the analysis of the microbiota of the oropharynx and gastrointestinal tract, will be performed on enrollment, at 1, 3, and 6 months (during probiotic consumption) and 1 and 6 months after the probiotic has been discontinued. The data obtained from this proposed study will advance knowledge relating to the safety of LGG and will provide a basis for multiple future studies to evaluate the safety, efficacy and mechanisms of action of LGG in vulnerable populations. Future studies will assess whether administration of LGG can influence oropharygneal colonization with Staphylococcus aureus in children with cystic fibrosis who are at risk of morbidity and mortality due to invasive Staphylococcus aureus pneumonia.